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Thermal evolution and present state of the ice shell of Europa

Europa could undergo gradual or rapid changes of heat production during its thermal history. Variations in heat production may have been caused by changes in the satellite's orbit over time (timescales 10^8 yr), with episodic thermal activity on the sea floor. The heat transfer from the silicate interior to the surface occurs in the ice shell by thermal conduction or possibly by stagnant lid convection. I investigate the response of the ice shell to changes of heat production in the interior of the satellite. Numerical experiments are performed to explore the behavior of a convective ice shell with strongly temperature-dependent viscosity both with and without internal heating. The Boussinesq fluid equations are solved using the ConMan finite-element code. The numerical model considers a Newtonian rheology for the water ice. I show that variations in heat flux, DeltaF, supplied to the base of a convective ice shell, cause large variations in the shell thickness, deltah. In contrast, for a conductive ice shell, large DeltaF involve relatively small deltah. For example, for a conductive ice shell 3.5 km thick, a 10% change in basal heat flux causes a variation of the ice thickness of roughly 0.9 km. In contrast, a convective ice shell with low Rayleigh number, 10% change in flux involve changes of thickness large than 10 km. For a convective fluid with temperature dependend vistosity, the heat flux jumps to a finite amplitude at the critical Rayleigh number. The implication fo this jump is that, for a range of heat fluxes, two ice-shell equilibria exist for a given heat flux, one comprising a thin, conductive shell and the other comprising a thick, convective shell. I show that modest variations in heat flux can force the system to switch between these two states, with consequent rapid changes in the ice shell thickness of 10-20 km. I carefully determine the conditions under which this can occur. Potentially this behavior of the ice shell has important consequences for resurfacing and tectonic processes on Europa. For example, refreezing of 10 km of hte ice shell would cause a radial expansion of Europa of about 1 km, which could produce surface fractures and transport deep materials to the surface. Furthermore, based on interpretations for how features such as chaos and ridges are formed, several authors have suggested that Europa's ice shell has recently undergone changes in thickness. The model provides a mechanism for this such changes to occur.